The invention relates to a method for determining the position of a punch of a powder press, preferably a metal powder press, with respect to a die.
For the manufacture of pressed parts which are accurate to size the positioning accuracy of the upper and lower punch of a powder press, for example a metal powder press is of importance. An accuracy of  less than 0.01 mm is to be maintained.
It is known to obtain a path measurement in that the path of the piston of a hydraulic punch, which actuates the punch, is acquired. On account of the forces occurring on pressing there occurs a resiliency of the machine frame. The measured values therefore no longer correspond to the actual ones. If the die is coniform, a deflection of the die table also occurs.
It is therefore the object of the invention to specify a method for determining the position of a punch of a powder press which functions accurately and is largely independent of deformations of the press on account of the pressing forces.
This object is achieved by the features of the present invention.
With the method according to the invention two path sensors are used. The upper and lower punch comprise a so-called path sensor system which may function in a conventional manner. It functions with a relatively low accuracy for the coarse traversing region, wherein a measuring accuracy of 0.01 mm is sufficient.
Furthermore with the method according to the invention an incremental path sensor is used which e.g. has a measuring accuracy of 0.25 xcexcm. This is actuated directly by the punch, for example via an extension arm on the punch. The measuring range of such an incremental transducer is relatively short. It is sufficient when the position determination is effected with the help of the incremental transducer shortly before the plunging of the punch into the die.
Furthermore it is essential that the switching-over from the one path sensor to the other path sensor with a high traversing speed, of e.g. 500 mm/sec, is effected without jerking, without position losses and without losses of accuracy. For this purpose in the computer in which the readings are processed a reference measure is stored. This reference measure gives the predetermined distance of the punch to the die upper edge or die lower edge. The measure may be obtained in that a suitable gauge is arranged between the punch and the die table and the punch is traversed against the gauge with a very low speed. Since the distance measure is known, in the computer it may be stored as a reference measure. In the computer further a so-called reference offset is stored. It is obtained in that the punch from the reference measure is traversed away from the die and the number of pulses are counted which are produced up to the reaching of a so-called index pulse. The indexing is effected with the help of the incremental path sensor. If therefore during the pressing procedure from the incremental path sensor which is only actuated near to the reaching of the reference offset, the index pulse is triggered, the distance between the punch and the die is known and the incremental path sensor may then with the continuation of the pressing procedure indicate the position of the punch. If the number of pulses, which results from the sum of the reference measure and reference offsets, has been covered, the number of the pulses following this corresponds to the respective position of the punch in the die bore. According to one formation of the invention in a counter the number of pulses which are produced with the traversing of the punch from the reference measure towards the reference offset is stored. In a second counter the respective actual position is acquired by the incremental path sensor. The sum of the reference measure and reference offset is stored in a buffer in front of the second counter. On reaching the index pulse the buffer content is transferred to the second counter for the purpose of the sole position determination by the incremental path sensor.
With the method according to the invention on the one hand the switching-over from the absolute to the incremental path sensor is effected in a controlled manner by the computation within the framework of its process cycle time without position loss. Before each switching-over procedure the reference value (reference measure+reference offset) for the second counter is continuously regenerated by the index pulse. Position losses or adulteration by way of electrical disturbing influences may in particular with continuous operation of the press be avoided and thus also corresponding reading errors.